It has been known to plate metal, metal mixtures, and alloys in particulate form on a metal substrate by applying mechanical force sufficient to cause adhesion between the plating metal particles and the surface of the substrate. The mechanical force necessary to cause such adhesion is achieved by placing the plating metal particles, a solid impaction media (e.g. glass or steel beads), materials which promote such plating, and a metal substrate in a rotating ball mill or a tumbling barrel. In this manner, the rotation of the ball mill or the tumbling of the barrel imparts kinetic energy to the impaction media which is transferred to the plating metal particles such that these particles are pounded into the surface of the substrate as a coating.
The early work in this field of mechanical plating was disclosed in U.S. Pat. Nos. 2,640,001, 2,640,002, Re. 23,861, 2,689,808, and 2,723,204 all to Clayton et al. Typically, these mechanical plating processes were undertaken in the presence of a plating liquid containing additives to improve the efficiency of plating and/or the quality of the metal deposited. These additives include surfactants, film-forming materials, antifoaming agents, dispersants, and corrosion inhibitors. Some of these materials are often added together to the plating liquid as a promoter chemical. For example, U.S. Pat. No. 3,460,977 to Golben discloses promoter chemicals with specific surfactants and organic acid materials for mechanical plating. U.S. Pat. No. 3,328,197 to Simon teaches utilizing promoter chemicals in the form of a solid cake or bar which contains a combination of mechanical plating promoter chemicals. As the mechanical plating cycle progresses, the bar or cake dissolves at a rate which provides optimal amounts of promoter chemicals to the mechanical plating process. U.S. Pat. No. 3,268,356 to Simon ("'356 patent") discloses incrementally adding the promoter chemical and/or the plating metal particles to the plating barrel in successive additions to optimize the density and uniformity of the plating metal coating over the entire substrate surface.
In U.S. Pat. No. 3,400,012 to Golben, the advantages of electroplating were sought to be achieved in a mechanical plating process. Such galvanomechanical plating was effected by adding to the plating liquid a "driving" or plating-inducing metal and an ionizable salt of the metal to be plated. The "driving" metal selected is one which is less noble than the plating metal or the metal of the metallic surfaces to be plated. For example, in mechanically plating tin onto steel washers, the plating metal is in the form of tin chloride, and the driving metal is aluminum powder.
U.S. Pat. No. 3,531,315 to Golben ("'315 patent") discloses performing a mechanical plating process in the presence of a strong acid. Prior to the '315 patent, agitation of the plating metal, the impaction media, and the substrate generally was conducted in the presence of weak organic acids such as citric acid. This required that the contents of the plating barrel be rinsed free of any strong acids used to clean or copper the parts before starting the citric acid-based plating process. With the process of the '315 patent, it is possible to conduct the mechanical plating process without need for intermediate rinsing steps, rendering the process extremely economical.
In German Patent No. DE 28 54 159 to Tolkmit, intermediate coatings such as the copper flash coating which is normally applied prior to mechanical plating is applied in a single-stage process from a slurry containing an intermediate coating metal and a final metal.
One form of mechanical plating produces a lighter weight, relatively thin coating of 0.1 to 1.0 mils thick. Another form of mechanical plating, known as mechanical galvanizing, results in the application of a thicker (i.e. from about 1.0 to 5.3 mils) and heavier (i.e. from about 0.7 to 2.5 ounces per square foot) mechanically-applied metallic coating. During the development of such mechanical galvanizing processes, it was found that enhanced adhesion of mechanical galvanizing coatings could be achieved by building up thin layers of mechanically plated metal.
U.S. Pat. No. 4,389,431 to Erisman ("'431 patent") adapted the process of the '315 patent to the incremental metal powder additions of mechanical galvanizing. This was achieved with two chemical promoter systems. The first is a flash promoter which coats the substrate with a thin adherent flash coating of a metal more noble than the plating metal prior to adding the plating metal to the system. The second continuing promoter is then incrementally added with some or all of the incremental additions of a finely divided mechanical plating metal, the layers of which are built up to effect mechanical galvanizing.
Although mechanical plating processes are disclosed as having applicability to any number of malleable metals, metal mixtures, or alloys, certain metals such as aluminum are more difficult to plate in this manner than others. Aluminum has an oxide surface layer which not only is difficult to remove prior to use of aluminum powder in a mechanical plating process but which also reforms readily during the process. This oxide coating inhibits the plating efficiency of aluminum. For example, in mechanically plating metal mixtures containing aluminum powder, it has been found that the aluminum deposits only in very small quantities compared to the other metal powders in the mixture. Similar problems have been encountered in mechanically plating titanium, magnesium, and mixtures containing these metal powders.
As a result of the difficulty in mechanically plating aluminum, other plating technologies had to be used when it was necessary to plate with aluminum. For example, U.S. Pat. No. 3,415,672 to Levinstein et al. coats iron-nickel-cobalt superalloys in a pack-cementation process in which titanium and aluminum are codeposited onto the substrate surfaces from a vapor state (1750.degree. F. to 2150.degree. F.) in the presence of either sodium fluoride, potassium fluoride, or ammonium chloride. In U.S. Pat. No. 3,503,775 to Austin, aluminum coatings are applied to ferrous metal substrates by electrostatic deposition, rolling, and heating. U.S. Pat. No. 3,577,268 to Whitfield et al. coats iron-nickel-cobalt superalloys with an aluminum-magnesium alloy by cementation, dipping, or binder-slurrying procedures.
Some attempts have been made to coat substrates with aluminum by mechanical plating. In U.S. Pat. No. 3,443,985 to Cutcliffe, microscopically smooth, non-metallic, non-ionizable balls are utilized as an impaction media in coating nails with an aluminum powder alloy of 50% aluminum, 45% tin, and 5% zinc. U.S. Pat. No. 3,754,976 to Babecki et al. sprays a mixture of aluminum and small solid peening particles onto a variety of surfaces at a high velocity. German Offenlegungschrift No. 3,011,662 to Tolkmit describes a mechanical plating process in which the oxide problems with aluminum are said to be obviated by mechanically plating in a liquid containing aluminum, a hydrazine derivative, a polymerization product of propylene oxide and ethylene oxide, and a corrosion inhibitor. The hydrazine apparently releases hydrogen which combines with oxygen residues in the liquid. The activity of aluminum powder is to be maintained by the polymerization product, and, if necessary, further additions of alkali fluorides or fluorosilicates. Although these discoveries permit coating with aluminum, such prior art techniques yield an aluminum coated substrate which is not resistant to corrosion. Due to this problem, the use of light-weight, mechanically-plated, aluminum coatings have been limited. Accordingly, a need exists for a procedure of mechanically plating with aluminum which yields a corrosion resistant product.